Well concave blocks are certainly not the problem.

Always when you import a level, you get meshes with lots of doors and windows, they are 100% concave and do not make any problem.

Your problem probably is that your take a block and manipulate its vertices in a way that the result is no 3D object anymore. For instance, take a cube and move one of its vertices outwards. This creates an impossible object because surfaces are bent and then you get errors in the map.

This can not happen with meshes that consist of triangles, because they are always possible.